1. Field of the Invention
The present invention relates to a solid propellant charge for a propulsion unit and to a propulsion unit equipped with such a charge.
The invention applies more particularly, albeit not exclusively, to a large propulsion unit for a space launcher vehicle.
2. Background Art
Space launching systems fairly often comprise large solid-propellant propulsion units, usually known in space technology as "boosters". These have variable sizes from ten to several hundred tonnes, depending on the launching systems.
In fact, a launcher vehicle requires a high level of thrust during the first part of its trajectory, and solid propulsion is, up to now, the most economical means for meeting this need, provided that there is the necessary knowledge and investment in equipment.
The largest propulsion units are usually produced in pieces (segments), in order to limit the amount of investment necessary. For example, large propulsion units are known which comprise three segments, namely a small segment of 30 tons and two large segments of 100 tons, or four segments, each having a substantially equivalent mass (approximately 125 tons).
It is known, moreover, that the law of thrust of a large propulsion unit involves complex dimensioning aimed at overall optimization of the launcher vehicle, and this always results in a high initial thrust level followed fairly often by a gap for the purpose of avoiding excessive dynamic pressures on the launcher vehicle which bring about a high level of thermal and mechanical loads and stringent steering requirements for controlling it.
Since the combustion of a solid-propellant engine takes place in parallel strata, the only means of conforming to the specification of the law of thrust is to produce a suitable initial charge profile having a large combustion area. In practice, the high level of output required over the first part of the trajectory is obtained by means of a specific cross-sectional profile, for example in the form of a star, which has the property of disappearing after the desired period of time.
In all the large segmented propulsion units, this profile is placed at the front of the engine (the upper part in the operating position). There are two alternative embodiments of said profile, in particular having a star-shaped cross section:
a specific smaller segment may be made in order to produce this profile more easily on an industrial scale (removal from the mold in a single operation), but this is to the detriment of performance (the presence of unequal segments makes it necessary to manufacture at least one additional segment), PA1 the profile may require a core which has to be demounted in pieces after casting, and this constitutes a source of increase in the recurring cost. PA1 the propellant charge profile of large combustion area may be provided in the rear part of the front segment of said charge, PA1 the propellant charge profile of large combustion area may be provided at the front part of the rear segment of said charge, or PA1 the propellant charge profile of large combustion area may be allocated to the rear part of a front segment and to the front part of a rear segment. PA1 if the profile of large combustion area is provided on the segment at the front of the charge, the profile is at the rear of said front segment, PA1 if the profile of large combustion area is provided on the segment at the rear of the charge, the profile is at the front of said rear segment.
Large "monobloc" propulsion units are also known, which derive from the latest techniques developed for military propulsion, the profile of these being placed at the rear. However, the largest "monobloc" propulsion unit being developed at the present time does not exceed 60 tonnes.